What is the name for when you’re writing a post and it suggests users if you type a name? (twitter/slack)

In twitter this happens when you input the '@' symbol and it then suggests users as you type more characters.

In Slack this also happens but without needing the '@' symbol.

So far I have heard of 'incremental searching' and 'search-as-you-type', but these refer to search boxes and not inline within a text area/post box.

What is this feature called and are there any resources out there that can help me create this feature myself?

Thanks.